Cryo-EM structure of the bifunctional secretin complex of Thermus thermophilus.

Edoardo D'Imprima,Lennart Kirchner,Ricardo Sánchez,Werner Kühlbrandt,Beate Averhoff,Ilona Rose,Ramachandra M Bhaskara,Ralf Salzer,Janet Vonck,Gerhard Hummer

doi:10.7554/elife.30483

Edoardo D'Imprima, Lennart Kirchner + Show 8 more

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https://doi.org/10.7554/elife.30483

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Abstract

Secretins form multimeric channels across the outer membrane of Gram-negative bacteria that mediate the import or export of substrates and/or extrusion of type IV pili. The secretin complex of Thermus thermophilus is an oligomer of the 757-residue PilQ protein, essential for DNA uptake and pilus extrusion. Here, we present the cryo-EM structure of this bifunctional complex at a resolution of ~7 Å using a new reconstruction protocol. Thirteen protomers form a large periplasmic domain of six stacked rings and a secretin domain in the outer membrane. A homology model of the PilQ protein was fitted into the cryo-EM map. A crown-like structure outside the outer membrane capping the secretin was found not to be part of PilQ. Mutations in the secretin domain disrupted the crown and abolished DNA uptake, suggesting a central role of the crown in natural transformation.

Highlights

Natural transformation is a major mode of horizontal gene transfer (Blokesch, 2017; Mell and Redfield, 2014), by which bacteria take up DNA directly from their environment
In previous studies we discovered a unique secretin (PilQ) complex in the thermophilic bacterium Thermus thermophilus, which is essential for natural transformation and extrusion of type IV pili (Friedrich et al, 2002; Schwarzenlander et al, 2009)
We showed that T. thermophilus PilQ contains a thermostable C-terminal secretin domain and an exceptionally long N-terminal tail of six stacked rings (N0-N5) (Burkhardt et al, 2011; 2012; Salzer et al, 2016)

Summary

Introduction

Natural transformation is a major mode of horizontal gene transfer (Blokesch, 2017; Mell and Redfield, 2014), by which bacteria take up DNA directly from their environment. In this way, bacteria gain novel genetic information, for example metabolic traits, pathogenicity determinants and resistance genes as a driving force for bacterial adaptation and evolution. Members of the secretin protein family are conserved key components of natural transformation systems in Gram-negative bacteria, and play important roles in protein secretion, type IV pilus extrusion and the assembly and extrusion of filamentous bacteriophages (Ayers et al, 2010; Korotkov et al, 2011). The OM-embedded C-terminal secretin domain likely provides an aperture for DNA and protein translocation through the OM, connecting the periplasm to the

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